Solid phase extraction (SPE) is a popular and growing technique for preparing samples for analysis. See Horack et al., Perspectives from the Leading Edge in Solid Phase Extraction, LC.GC 11, No. 2, p. 7490 (February 1993). Sometimes referred to as liquid solid extraction, SPE typically involves use of a membrane or other medium that has sorptive properties. A fluid is passed through the SPE medium, and various substances in the fluid are extracted onto the SPE medium by being sorbed thereon. The extracted substances are subsequently removed from the SPE medium in a concentrated form by, for example, desorption with an elution solvent. The extracted substances can then be analyzed to determine composition, concentration, et cetera. SPE is rapid, less labor intensive than conventional liquid/liquid extractions, and it eliminates, or at least minimizes, the need for large volumes of costly solvents which must be disposed of in an environmentally sound manner. Some of the many examples in which SPE is used include: determining trace amounts of pesticides in river water; analyzing pollutants in waste water; isolating organic compounds from groundwater; and testing urine samples.
Although SPE has been met with increased popularity in the analytical art, the method has a significant drawback: it is difficult to carry out SPE on a fluid that contains suspended solids. In attempting to perform SPE on such a fluid, the SPE medium's pores become plugged with the solid particles as the fluid is passed through the SPE medium. Pore plugging slows passage of fluid through the SPE medium and can render the SPE process extremely time consuming; see Markell et al., New Technologies in Solid Phase Extraction, LC.GC, 9 No. 5 (1991); and O'Donnell et al., Proceedings from Seventh Annual Waste Testing and Quality Assurance Symposium, 2, 182-194, Washington, DC (Jul. 8-12, 1991). The pore-plugging problem is particularly problematic for fluids that contain large quantities of suspended solids, for example, river water.
Several approaches have been suggested in the SPE art to overcome the slow flow rates caused by suspended solids in a fluid. Markell et al. (supra) have suggested using a smaller sample and detecting the analyte in the extract by more sensitive analytical methods, and if a larger sample needs to be processed, Markell et al. have suggested using a pre-filter to remove the suspended solids; see also supra O'Donnell et al. Another approach taught by Markell et al. involves allowing the sample to settle, and then performing SPE on the clear layer of the sample. It also has been suggested to pretreat the fluid with an acid to dissolve certain solids such as insoluble salts. See Hagen et al., Membrane Approach to Solid Phase Extractions, Analytica Chimica Acta, 236 157-164 (1990).